Beam directors in three dimensional (3-D) printers and scanners contain galvanometer servo motors and linear actuators to drive and direct mirrors and crystals in order to deflect and direct beams. The printing and scanning speed is therefore limited mainly by galvanometer and actuator speed.
A galvanometer servo motor is limited to max scan speed of about 2.5 KHz. Galvanometer servo motors also have about 5-10 micro radian positioning error. This error becomes more prominent as the target distance from the galvanometer servo motor driven mirror increases. In addition a galvanometer servo tends to shudder when it reaches its destiny and therefore presents settling down unwanted noise.
Linear actuators can be used to eliminate galvanometer errors. However if linear actuators are used then their full forward and backward speed cycle is limited due to slow acceleration and deceleration caused by their inertia.
Another common method of Laser scanning and printing is the use of polygon mirrors. Polygon mirrors can be used to direct the beam in one dimension, while the second dimension can be implemented by a linear actuator or a galvanometer. Although, Polygon mirrors improves on the galvanometer speed limitation, they will contribute additional distortion due to the geometry of the mirrors while non-linear mapping of the beam from the input to the output field takes place. In addition, all polygon mirrors must be completely identical. Both the X-Y-axis galvanometer and polygon mirrors techniques suffer further distortions due to the f-theta lens imperfection. The use of f-theta contributes two additional errors:
1. The beam angle to the normal of the surface will grow as it travels away from the center of the lens, causing an elliptic like beam formation instead of a circle.
2. The optic conversion errors of f-theta will grow as the beam travels away from the center of the lens; the optic conversion of tan(theta) will grow non linear as theta grows.
The object of this invention is to mitigate the problems discussed above.